The very existence of El Niño - the oscillatory behavior of the tropical
Pacific climate - may be due to the warmth of the tropics (relative to the
coldness of the high latitudes). This is elucidated by subjecting a
mathematical model for the coupled tropical ocean-atmosphere system to a
varying radiative heating. The temperature of the deep ocean is kept fixed. In
response to an increasing radiative heating, the coupled system first
experiences a pitch-fork bifurcation that breaks the zonal symmetry imposed by
the solar radiation. The resulting zonal sea surface temperature (SST)
gradients increase with increases in the radiative heating. When the zonal SST
gradients exceed a critical value, a Hopf bifurcation takes place which brings
the system to an oscillatory state, a state that closely resembles the observed
tropical Pacific climate. Further increases in the radiative heating result in
increases in the magnitude of the oscillation. The results shed new light on
the physics of El Niño and suggest that climate change due to
anthropogenic forcing may occur through the same dynamic modes that sustain
natural variability.